1. Field of the Invention:
The present invention relates to a spring steel having a good durability and a good sag-resistance.
2. Description of the Related Art
There has been an increasing demand for light weight suspension coil springs reflecting a trend toward light weight parts in transportation such as automobiles in order to save energy and realize high performance, in recent years.
An effective approach to the reduction of weight is to design the springs to have an increased stress, in other words, to increase a designed stress. Designed stress referred to here is defined as a stress required of the springs in design. However, if conventional spring steels are used to produce coil springs for vehicles, having an increased designed stress, there will be problems such that the level of the springs will be lowered as time passes (it is so-called sag), accordingly the height of the vehicle will significantly be decreased, and consequently the location of the bumper will be lowered thus leading to a serious problems for safety. Accordingly, it has been impossible to increase a designed stress in the springs.
In the use of the springs, a pulsating load is repeatedly applied thereto. When the designed stress is increased, the springs would be broken in the early stage.
In view of the abovementioned problem, it has been strongly desired to develop a spring steel which is excellent in both a sag-resistance and a durability.
As a conventional coil spring, JIS SUP6 had been used. However, it has become evident that Si is effective in sag-resistance, JIS SUP7 has broadly been used. A spring steel containing at least one of vanadium and niobium, in JIS SUP7 has been developed and is used at present, as a spring steel which is excellent in a sag-resistance and capable of weight reduction.
However, there is a stronger demand for light weight automobiles, so it is desired to develop spring steels having a performance superior to that of the aforesaid spring steel containing vanadium and/or niobium in JIS SUP7, and having more excellent sag-resistance and durability which enable the use under a higher stress state.
In a conventional spring steel, a method to increase the hardness of a spring has been used for the purpose of using it under a high stress state. Though this method can improve the sag-resistance, deterioration of the durability due to toughness deterioration was inevitable. The deterioration of the toughness causes increase of notch sensitivity. Accordingly, a brittleness breaking which starts from an inclusion or flaw which exists inside a material, easily occurs by repeated stresses lower than a allowable stress. The durability of a spring is significantly reduced. Under the circumstances, there is a stronger demand for a spring steel which makes high stress designing possible.